GAW Report No. 268, 2021: Smit, H.G.J., and Thompson, A.M., and the ASOPOS 2.0 Panel, "Ozonesonde Measurement Principles and Best Operational Practices. ASOPOS 2.0 (Assessment of Standard Operating Procedures for Ozonesondes)", WMO Global Atmosphere Watch Report Series, No. 268, World Meteorological Organization, Geneva